In order to improve design or decorative property or to impart excellent touch or texture (e.g., soft texture), there have been proposed that composites (molded composite articles) formed from a combination of a plurality of resins each having a different hardness, for example, a molded composite article in which at least part of a resin molded article is coated with a thermoplastic elastomer. Such a molded composite article is usually produced by adhesion of a plurality of molded members through an adhesive. For example, Japanese Patent Application Laid-Open No. 267585/1996 (JP-8-267585A) (Patent Document 1) discloses a resin molded article in which a plurality of resin molded articles formed from a polyamide resin or others are adhered to each other through a finishing agent such as a urethane polymer or a urethane adhesive. However, such a process using an adhesive is not only uneconomical due to complicated steps, but also has problems such as environmental pollution by an organic solvent or others.
On the other hand, from the viewpoint of rationalization of production processes or environmental protection, a process for thermal welding of a plurality of molded members has been adopted. The molded composite article obtained by thermal welding is usually manufactured by a molding process such as a two-color (or double) molding or an insert molding in many cases. However, combination of different materials subjecting to thermal welding is significantly limited. Moreover, it is not easy to establish molding conditions to ensure enough bonded strength.
Therefore, in addition to thermal welding, the fused part is reinforced by a combination use of a process for creating a concavo-convex site (or part) in the composite area of the molded member to bond mechanically, or a process for coating a primer or others on the bonding (or fusing) part, or other methods. In such a method, however, the molded composite article is deteriorated in flexuous property. For example, the hardened primer layer easily forms a crack by bending. Moreover, the production process needs to complicate the structure of the molded member, resulting in increase of the number of the production steps.
In order to solve these problems, it has been investigated to use a thermoplastic polyurethane as a material for a resin member constituting a molded composite article. The thermoplastic polyurethane itself is relatively excellent in adhesiveness. For example, in an application for shoe(s), a molded composite plastic article comprising a polyamide resin and a thermoplastic polyurethane is practically used as a shoe sole. For example, Japanese Patent Application Laid-Open No. 505333/1996 (JP-8-505333A) (Patent Document 2) discloses that a lightened shoe sole is obtained by inserting or putting a molded article made of a thermoplastic resin such as a polyether amide, a polyether ester or a polyurethane in a mold, injection-molding a polyamide elastomer containing a foaming agent into the mold, and adhering the thermoplastic resin molded article (un-lightweight (un-lightened) plastic) to the elastomer (lightweight thermoplastic elastomer).
In such an application for shoe sole, it is preferred to use a member composed of a thermoplastic polyurethane resin excellent in flexibility in terms of securing flexibility of the whole shoe sole. Moreover, as the counterpart member of the polyurethane resin, a member composed of a polyamide elastomer excellent in flexibility is frequently selected. In the compounding process, both members are generally subjected to thermal welding in the molding process. In these cases, the polyamide elastomer composed of a polyamide block copolymer having a polyether segment in a molecule thereof is combined with the thermoplastic polyurethane having a polyether segment in a molecule thereof. Thereby, affinity between polyether segments in common with both resins is attributed to relatively easy thermal welding therebetween.
However, in this method, water management of the materials in the molding process (usually an injection molding) is difficult, and moisture absorption of the material significantly decreases the bonded strength therebetween. Further, the bonded strength is also influenced great deal by the temperature of the resin in the molding process. In the case of insert-molding either of the members, it is possible that the member composed of the polyurethane resin is inserted followed by injecting the polyamide elastomer thereon (overmolding the polyamide elastomer). However, in the reciprocal case, enough bonded strength is unobtainable. The polyamide elastomer is high in heat resistance and can be heated up to a sufficiently high temperature (e.g., 250° C.) in the injection molding, but the polyurethane can be heated only up to about 200° C. because of poor heat resistance thereof. The reason is that the melting point of the polyamide elastomer is generally higher than that of the polyurethane elastomer. Accordingly, the above combination had the problem that economically disadvantageous multi-color molding machine was forced to be employed.
Further, as mentioned above, since the bonding of this combination depends on affinity between polyether segments in common with both resins, it is drastically difficult to combine a polyurethane having a small content of the polyether segment, particularly a costly advantageous polyester-series polyurethane, with a polyamide elastomer, and therefore commercially disadvantageous factors have been severely left.                Patent Document 1: JP-8-267585A        Patent Document 2: JP-8-505333A        